This invention relates to a light source for atomic emission spectroscopy and more particularly to a glow discharge device adapted to be used as a furnace for atomic emission spectroscopy.
One technique which exists for vaporizing and atomizing a sample to be analyzed using atomic emission spectroscopy with glow discharge excitation is known as furnace atomic non-thermal excitation spectrometry (FANES). This technique uses resistive heating of a graphite cathode to achieve furnace action to vaporize a sample. This technique requires a large (2.5 Kw) power supply to resistively heat a graphite furnace of the type used in atomic absorption spectroscopy. The large size of the power supply required with this technique inherently makes the FANES device heavy and awkward to use. One of the most difficult problems associated with analytical devices utilizing atomic emission spectroscopy is the separation of the spectral signal due to the analyte from the background light spectra of the ion plasma.
Accordingly, it is an object of the present invention to provide a glow discharge furnace for use in atomic emission spectroscopy using a relatively small power supply making the present invention therefore more economical. It is another object of the present invention to provide an analytical apparatus that is more sensitive to analytes than has previously been attainable.
The preferred embodiments of the present invention include an ion bombardment furnace comprising a magnetron glow discharge lamp having a closed cylindrical chamber containing an ionizable gas and having a center post cathode therein. The lamp in one embodiment is physically located between poles of an electromagnetic so that a magnetic flux field passes through the chamber coaxial with the cathode. In another embodiment, the magnetic field is provided by a tubular stack of ring shaped permanent magnets arranged around the cathode. In both embodiments, the magnetic flux field is directed coaxially with the cathode and a magnetic field of approximately 1.25 kG is typical.
The chamber includes a transparent window in the outer wall for passage of light emitted by excited atoms. The apparatus according to the invention includes focusing mirrors which focus the emitted light and direct this light into a spectrometer which separates the emitted light into a spectrum. A grating or other dispersive element within the spectrometer is then set to pass a narrow bandwidth of wavelengths to a photoelectric detector. The detected light of the selected wavelength is then converted into an electrical signal which is proportional to the intensity of the detected light. The electrical signal is then sent to a display device.
The chamber also has a closable sample access port through which an analyte solution may be inserted and deposited onto the cathode. In addition, the chamber includes a gas inlet and outlet for purging the chamber volume with an ionizable gas such as argon. Positioned concentrically around and spaced from the cathode inside the chamber is a ring shaped anode. The cathode and anode are connected externally to a DC power supply sufficient to cause initial ionization of the argon gas within the chamber initially forming a plasma.
Operation of the ion bombardment furnace for atomic spectroscopy in accordance with the present invention will now be described. An analyte solution is first micropipetted through the access port into the chamber and deposited onto the cathode. The chamber is evacuated and then flushed with argon gas. A pressure of approximately 1 to 5 Torr of argon is then maintained. An electrical current is supplied to the cathode by applying a direct current (DC) potential between the cathode and anode setting up a radial electric field to cause initial ionization of the argon gas and formation of the plasma about the cathode. This DC potential also causes a cathode current to initially heat the cathode.
The presence of the magnetic field retains the electrons formed in the plasma in the proximity of the cathode and enhances the bombardment of neutral atoms of the argon gas. This in turn enhances the production of positive ions. These positive ions then bombard the cathode causing rapid further heating of the cathode and rapid vaporization of the analyte.
The vaporized analyte is excited in the glow discharge plasma. Characteristic light is then emitted by the electrons in the excited atoms as the electrons return to a lower energy state. This light is passed out of the chamber, focused, spectrally separated in the spectrometer, and selectively detected by a photomultiplier tube detector positioned about the exit slit from the spectrometer.
The photomultiplier tube detects light at the selectively passed wavelength and converts this detected light to an electrical signal proportional to the intensity of light emitted. The high signal to noise ratio achievable with this arrangement suggests an absolute limit of trace element detection in the analyte in accordance with this invention in the picogram range.
Other features and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subject description of the invention and the appended claims taken in conjunction with the accompanying drawing.